Water and carbon dioxide are similar because they both contain at least one oxygen

Yes that is true.
Water:
Carbon dioxide:
Both have O

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).

Answerhund's rule;